Ion exchange membranes having a macroporous surface area

ABSTRACT

THIS INVENTION IS DIRECTED TO ELECTRODIALYSIS EQUIPMENT PARTICULARLY TO COMPONENTS COMPRISING THE ION EXCHANGE MEMBRANES. THESE MEMBRANES ARE FABRICATED WITH A POLYMERIC MACROPOROUS SURFACE AS CONTRASTED TO THE PRIOR ART MICROPOROUS &#34;GEL&#34; TYPE MEMBRANES. THE MACROPOROUS MEMBRANE SURFACE OR LAYER LESSENS THE TENDENCY FOR MEMBRANES ESPECIALLY THE ANION SELECTIVE TYPE TO FOUL WHEN EMPLOYED IN THE ELECTRODIALYSIS OF SOLUTIONS CONTAINING FOULING CONSTITUENTS. THE MACROPOROUS MEMBRANES ARE OBTAINING BY POLYMERIZING THE LIQUID MONOMERS IN THE PRESENCE OF A NON-POLYMERIZABLE SOLVENT PAIR OF DILUENT COMPRISED OF BOTH A &#34;GOOD&#34; AND A &#34;POOR&#34; SOLVENT, SAID SOLVENT CHARACTERIZED WITH RESPECT TO THE SOLVENTS ABILITY TO SWELL THE CROSSLINKED POLYMER THAT IS FORMED.

United States Patent 3,749,655 ION EXCHANGE MEMERANES HAG A MACROPORGUS SURFACE AREA Russeli B. Hndgdon, J12, Sudhury, Mass, assignor to Ionics, Incorporated, Watertown, Mass. No Drawing. Filled lune 21, 1971, Ser. No. 155,357 lint. (Cl. Mild 13/02 US. Cl. 204-180 I? 3 Claims asset or THE mscros This invention is directed to electrodialysis equipment particularly to components comprising the ion exchange membranes. These membranes are fabricated with a polymeric macroporous surface as contrasted to the prior art microporous gel type membranes. The macroporous membrane surface or layer lessens the tendency for membranes especially the anion selective type to foul when employed in the electrodialysis of solutions containing fouling constituents. The macroporous membranes are obtaining by polymerizing the liquid monomers in the presence of a non-polymerizable solvent pair or diluent comprised of both a good and a poor solvent, said solvent characterized with respect to the solvents ability to swell the crosslinked polymer that is formed.

This invention relates to homogeneous, highly porous, cross-linked polymeric structures having at least two dimensions in excess of one cm. and especially to ionpermselective membranes having a microporous and/ or a macroporous polymeric structure, to the process of producing said membranes and to the electrochemical cell for employing the same. More particularly, the invention is directed to ion-selective membranes having at least one major face or surface comprised of a macroporous (macroreticular) resin with the remaining portions of the membrane comprised substantially of the conventional microporous gel type resin. Specifically it is directed to the prevention or overcoming of the fouling of ion-selective membranes during the electrodialysis of aqueous solutions containing large organic or inorganic molecules which normally foul conventional gel twpe microporous membranes.

The electrodialysis of aqueous solutions (especially those containing organic vegetable matter such as surface waters, sewage material, sugars, pharmaceuticals etc.) frequently suffer from membrane fouling which is especially noticeable with the strongly basic types of anion selective membranes which contain ion-exchange groups of quaternary ammonium, quarternary phosphonium, tertiary sulphonium etc. Fouling however is still a problem with all present day membranes even those made with resins containing weakly basic groups although not to the same extent.

These fouling substances present in the solution being electrodialyzed are evidenced during operation by a considerable increase in the resistance (voltage) of the membrane demineralizer over a period of time. This increase in electrical resistance is believed due to the precipitation, clogging and/or imbedding of the membrane surface by medium and large molecular weight organic compounds and/ or insoluble salts which originate from the solution being electrodialyzed. Fouling of membranes is encountered for example especially in the electrodialysis of industrial solutions such as sugar solutions, red wine etc. which contain substantial amounts of the offending material. The fouling material becomes lodged or formed in situ within the membranes or will tenaciously cling to their surfaces thus building up a dielectric layer. Electric membrane demineralizers of the type employing ionexchange membranes are well known in the art and are fully described in the U.S. Pat. Nos. 2,708,658, 2,826,544, 2,848,403, 3,239,442, 3,341,441 and many others.

Several prior methods of alleviating membrane fouling have been attempted with only minor degrees of success. These methods include the addition of acid to the solution to be treated, pretreatment of the solution for removal of the offending molecular or polyvalent ions as by clarification or defecation by use of charcoal, carbon, granular ion-exchange resin etc. or the periodic or cyclic reversal of the direct current employed during operation of the electrodialysis apparatus. Other methods involve removing the fouled membranes from the electrodialysis stack and treating with chemicals such as acids, salts etc. or sanding or wire brushing the surfaces of the membranes to remove the fouled layer. These latter methods of treating the membranes directly will recover them for further use until fouling again reoccurs wherein a repeat dismantling of the apparatus is again required. Eventually however the membranes are not recoverable and must be replaced.

It has now been found that the fouling of membranes under the circumstances described hereinabove may be partially overcome and alleviated by the use of ion-selective membranes having a rigid macroreticular (macroporous) structure located at least on the surface area side of the membrane which is being subjected to the fouling constituents during the electrodialysis process. The macroreticular resin has a large surface area since it is highly porous, in that it contains pore diameter sizes preferably from 500-1500 A. (angstroms). The remainder of the membrane is preferably of a polymer composition comprised substantially of the conventional homogeneous gel type microporous membrane resin such as disclosed in US. Pat. Nos. 2,636,851, 2,730,768, 2,731,441, 2,860,091 and others having the usual pore diameter sizes from 10-30 A. Although membranes can be fabricated having a totally homogeneous macroporous structure throughout that is pore sizes of from 500-1500 A. dispersed throughout the polymer, it is preferred to employ the macroporous surface type membrane for the electrodialysis process. Such a membrane will allow only its surface to accumulate the organic foulants While the membranes interior comprised of the conventional micropore size resin will not develop changes in its electrical resistance, current efficiency, water content or leakage of solution therethrough. As is well known in the resin art, macroporosity can definitely be determined by the use of electron microscopy and mercury porosimetry and it is further revealed in the specific surface area of the resin as measured by nitrogen absorption.

It is therefore, the primary object of this invention to manufacture crosslinked macroporous polymeric structures measuring at least one centimeter in two dimensions for use as intermediates in the production of ion-permselective membranes of both the cation and anion type.

Another object is to make electrically conductive ionselective membranes or sheets containing at least one surface of macroporosity with the remaining portions of the membrane having a gel type microporosity.

A further object is to utilize macroporous polymeric ion-selective membranes as hydraulic separators in electrolytic and electrodialysis equipment in the treatment of solutions containing membrane fouling constituents.

Other objects and advantages of the present invention will become apparent to one skilled in the art from the following description and appended claims.

The macroporous polymeric matrix or skeleton suitable for use as the intermediate structure in the manufacture of novel, ion-selective membranes can be prepared by copolymerizing a variety of monomers which are well known in the art under conditions that will yield a polymeric board structure containing at least one surface area of macroporous resin. For purposes of this disclosure a board is the sheet or film of a highly cross-linked polymeric structure prior to the introduction of ion-exchange groups therein, which groups give the polymeric resin the ability to act as an ion-permselective membrane. It will be apparent that the boards are not endowed with ionexchange characteristics of either anion or cation charge until such ion-exchange active groups have been attached thereto. The boards, however, are the base or intermediate material from which either cation or anion-exchange membranes may be obtained by the well known chemical treatment of the same to introduce known positive or negative ion-exchange groups. Reinforcement of the board may be provided by one or more layers of a support material which is not generally subject to chemical attack such as woven glass (Fiberglas), polytetrafiuoroethylene (Teflon) polypropylene, polyethylene, and the like, on which the polymer is cast. The board itself is prepared for example by copolymerizing a monomeric compound having one polymerizable double bond such as a monovinyl compound of styrene with a cross-linking monomeric compound having at least two polymerizable double bonds for example divinyl benzene. An important factor in the fabrication of the membranes of the present invention is the use of a pair of inert non-polymerizable (NP) organic solvents into which the double bond monomeric compounds are first diluted prior to the polymerization. This inert solvent pair should be present during polymerization to the extent of at least by volume based on the total volume of the monomeric mixture including the solvents. Although a minimum total solvent content of about 10% has been found effective for purposes of this invention, preferred embodiments include much larger amounts between about 2050%. Structures including as much as a total of 75% solvent have been found to be quite satisfactory. The solvent pair should comprise a good solvent and a poor solvent which terms will be more specifically defined hereinafter. During the polymerization of the monomers there is a natural tendency for the polymerizate to reject a poor solvent while dispersing a good solvent although both solvents are initially fully miscible with the starting liquid monomeric mixture. This makes possible the imparting of a macroreticular (macroporous) surface to a polymeric board while simultaneously allowing the preparation of the conventional micropore structure to the remaining portions of the board. The monomeric mixture containing the solvent combination of at least one good solvent and at least one poor solvent" is seeded with a small quantity of a polymerization catalyst such as benzoyl peroxide. The volume ratio of poor to good solvent although critical can be varied to some degree provided however that the poor solvent comprises at least about to 55% of the total solvent content. Naturally the greater the surface thickness desired of the macroporous layer the more of the poor solvent required. The macroreticular layer of the polymeric board can be as small as 0.1 mil thickness to possess anti-fouling characteristics with no limit to the maximum thickness since the entire membrane can be made substantially macroporous if so desired by employing the macroporous pore forming poor solvent as the predominate solvent in total monomer diluent employed. The total membrane or board thickness should be from 1 to 100 mils (preferably 10 to 25 mils) with the macroreticular resin surface layer being preferably at least about 10% of that thickness, that is from 0.1 to 10 mils for each macroporous surface layer.

For purposes of this disclosure good solvents are those having boiling points which are higher (preferably at least 10 C. higher) than the polymerization temperature employed, should be inert to the polymerization reaction (in that they do not react chemically or polymerize with the monomers) should preferably be totally miscible with the starting liquid monomers and should act as a swelling agent for the resulting solid copolymer by being absorbed or imbibed therein. The use of various good solvents for membrane fabrication is fully disclosed in U.S. Pat. Nos. 2,730,768, 2,731,441, etc. and include for example liquid aromatic hydrocarbons such as toluene, xylene, naphthalenes, benzene, and diethylbenzene; ketones such as cyclohexanone, methyl ethyl ketone and acetone; ethers such as diethylene glycol dimethyl ether (diglym), dioxane or dichloro diethyl ether; halogenated hydrocarbons such as methylene chloride, propylene chloride, chloroform, carbon tetrachloride, dibromethane, ethylene chloride or ethylene bromide. Also suitable mixtures of good solvents in preference to a single good solvent may be employed whenever desired. Other good solvents may be used besides those specifically mentioned above which are susceptible of forming solutions of the required concentration in order to impart microporosity to the copolymer and which does not interfere with the polymerization. The volume of good solvent present during polymerization determines the percent microporosity and substantially fixes the solvent or water holding capacity or content of the resulting polymeric structure. The good solvent contained in the polymeric structure can be replaced by another solvent, and the structure will imbibe about the same volume of water or other liquid as was present or original good solvent during the polymerization reaction.

Homogeneous microporous selective membranes may be produced from a polymeric matrix structure resulting from combining a polyvinyl aromatic compound such as divinyl benzene with a monovinyl aromatic compound such as styrene or ethyl styrene diluted in a good solvent only and polymerized in the presence of a catalyst. The resulting homogeneous microporosity of the structure can be controlled somewhat by varying the quantity of the good solvent within the monomeric mixture.

A poor solvent on the other hand is one that is also chemically inert under polymerization conditions and miscible with the monomeric mixture but of greater importance it must be incompatible with the resulting polymer or copolymer, that is, failing to substantially swell or become dissolved in the polymer. The poor solvent must not exert any substantial solvent action on the copolymer or be imbibed by the copolymer.

The determination of the most effective poor solvent and the amount required for the formation of a particular surface thickness of a macroporous copolymer structure may vary from case to case. Although there is no universal or single class of poor solvents applicable to all cases, the requirements of solubility with the starting liquid monomeric mixture and non-solubility in the resulting solid copolymer such as polystyrene can be tested empirically. It has been experimentally determined that surface macroporisity will develop in the board structure if a minimum concentration of poor solvent be present of about 15 volume percent of the total solvent concentration to effect the required phase separation. The liquid system containing the solvent pair of a good and a poor solvent remains substantially homogeneous and will not separate when the poor solvent is present in small amounts, that is below the critical concentration of about 15%. The minimum concentration of poor solvent in the polymerizing mixture should be in excess of this critical concentration and this excess amount can be varied so as to modify the extent and thickness of the macroporosity of the resin surface. As the concentration of liquid monomer in the polymerizing mass decreases (due to the polymerization) and as the concentration of the resutling solid copolymer increases, the poor solvent is repelled out of the polymeric phase passing necessarily in the direction of the polymer surface where it tends to accumulate if prevented from leaving. This explosion of the poor solvent towards the resin surface results in the formation of a structure having the larger pores (500- 1500 A.) located towards the resin surface area with the separate and distinct micropores (1030 A.) located primarily at the deeper portion of the resin, that is away from the surface. Particularly satisfactory poor solvents for preparing a macroporous copolymer such as a polymer resulting from a styrene-divinylbenzene monomeric mixture are the lower alcohols (methyl, ethyl, propyl and butyl) and certain of their isomers; also certain nonaromatic hydrocarbons solvents which includes hexane, cyclohexane, heptane, isooctane and the like as well as mixtures of the same. Also useable are the glycols such as polypropylene and polyethylene glycols or mixtures of the same, polycarboxylic acids, the lower monobasic organic acids, kerosene and the like. The use of these poor solvents will produce a macroporous polymeric structure having a whitish and opaque appearance which is characteristic of solvent rejection by the polymer phase. These resulting polymers will retain their macroporous structure even after being converted into ionpermselective membranes by well known techniques as by the introduction of cation groups such as sulfonic acid, carboxylic acid etc. or by the introduction of anion groups such as amines etc. Anion groups can be introduced for example by chloro-methylating the polymeric board structure and then aminating to introduce the quarternary amonium groups. Cation groups can be introduced for example by sulfonating the board structure by reacting with a sulfonation agent such as concentrated sulfuric acid, oleum, chlorosulfonic acid etc. Other suitable ionic acidic ion exchange reactive groups (cationic) are those generally used in ion exchange reactions, e.g. phosphonic, phosphinic, thiophosphinic, arsonic and carboxy. Other suitable ionic basic ion exchange groups (anionic) also known from the art include primary, scondary or tertiary amino groups, phosphonium, tertiary sulphonium groups and the like.

The membranes may be formed by various methods but it is preferred that a polymer substrate of liquid monomers be polymerized in sheet form between confining glass plates and then post-treated to introduce cation or anion exchange groups as the case may be. Of course the post treatment is not necessary where the monomer employed is already in the exchange group forms as where the monomers themselves contain cation groups such as vinylsulfonic acid or methyl styrene vinyl phosphoric acid and the like. The macroporous surface boards are preferably formed on reinforcing or supporting sheet fabrics by placing one or more sheets on a fiat bottom casting surface on top of one another. The mixture of the good and poor solvent pair and the polymerizable monomeric components or partially polymerized components are poured over the support material, covered with a flat top casting surface and then mass-heated until polymerization is complete. For example the monomer mixture may be poured into a rectangular tank into which glass plates and a reinforcing web or mat of an inert material such as glass cloth, polypropylene etc. is alternately placed in a stack arrangement and totally covered with the monomeric-solvent mixture. Thus there is between each pair of smooth glass plates a reinforcing sheet material completely saturated with the monomeric liquid mix. Thus the parallel glass plates are spaced from each for a distance substantially equal to the thickness of the reinforcing sheet. The tray is then heated and the liquid mixture allowed to polymerize. During the polymerization process the poor solvent in the mix is rejected by the forming polymer (phase separation) but being unable to escape the glass barrier it will preferentially and out of necessity locate itself at the general interface area between the glass plate and the polymer adjacent thereto.

The polymer matrix or board which acts as the skeleton for the various functional exchange groups of the novel macroporous membrane of the present invention are preferably based upon a styrene-divinylbenzene copolymer and may be varied as to their chemical structure over a considerable range as is well known in the art of making membranes. For example the monovinyl aromatic hydrocarbon monomeric component of the board structure can be employed individually or as mixtures and may be styrene (vinyl benzene) or its nuclear and/or alpha substituted derivatives such as ethyl vinyl benzene (ethyl styrene), vinyl toluene (methyl styrene) and its isomers, such as isopropyl styrene, also chlorostyrenes, paramethylstyrene, cyanostyrene, methoxylstyrene, acetyl styrene, dimethylstyrene and the like.

The polyvinyl aromatic monomer which furnishes the cross linking groups to produce an insoluble resin may comprise divinyl benzene (DVB), and substituted derivatives thereof such as the nuclear and/or alpha substituted derivatives such as vinyl toluene, divinyl toluene, dimethyl divinyl benzene, dimethyl divinyl toluene, and the like. Also useful are trivinyl-benzene, trivinylnapthalene, ethyl acrylate, vinyl acetate, ethylene glycole dimethacrylate etc. The divinyl benzene of commerce usually contains a large fraction of ethyl vinyl benzene (a non cross-linking agent) and also a small amount of non-polymerizable solvent of diethyl benzene. The highest divinyl benzene content of todays commercial product is about 72% but this is expensive and difficult to obtain, therefore necessitating the use of the more readily available 50-60% DVB. The amount of the cross-linking agent employed may vary within wide limits from 20-80 mole percent of the total polymerized monomers in the membrane resin phase with the preferable range being between 25-50 mole percent.

The copolymerization of the monomers is accelerated by means of Well recognized catalysts such as organic peroxide compounds (benzoyl peroxide being the preferred reagent for this invention) azo catalysts and the so called redox free radical catalysts such as perborates and persulfates which are suitably activated by appropriate reducing species. The monomers may also be suitable copolymerized by the well known art of irradiation whether the source is light or the more effective gamma radiation waves.

Having described the invention in general terms, the following examples will more particularly illustrate the invention.

EXAMPLE I (Totally macroreticular membrane) 55% divinylbenzene ml 1 396 Styrene (B.P.=145 C.) ml 117 Diethylbenzene (B.P.=18ll84 C.) ml 97 Iso-butyl alcohol (B.P.=l08 C.) ml 390 Benzoyl peroxide grams 4.7

1 Contained 12 m1. of diethylbenzene.

The above clear liquid monomeric mixture is placed into a 3" deep polymerization tray having dimensions of 12 x 10 /2" and alternately sheets of glass plates (9" x 9") and woven Dynel (T.M.) cloth (0.055 cm. thick) are stacked into the mixture (sandwich style) until a total of nine pieces of glass plates alternated with eight pieces of cloth. This tray, with its contents are placed into an explosion proof oven overnight with an initial oven temperature setting of 60 C. rising to C. over the first three hours and then maintaining a polymerization temperature of 75 C. for the balance of a 15 hour period.

At the end of the polymerization period, the solid mass of polymer is released from the tray and after cooling, the glass plates are removed one at a time with concurrent release of the cast polymeric boards which are comprised of bulk polymer encasing the cloth fabric to form a reinforced sheet structure.

The boards are immersed in diethyl benzene and are white and very opaque in appearance in contrast to the clear gel type (non-macroreticular) boards or mem branes described heretofore in Clarke Pats. Nos. 2,730,768 and 2,731,441. The opaque appearance throughout is indicative of a substantially totally macroreticular resin structure.

(A) ANION EXCHANGE MEMBRANES Four of the above resulting boards are placed into ethylene dichloride for an overnight period. After this resin swelling step, they are immersed (by suspension) in a six liter tank containing a solution made up of four liters of chloromethyl ether, one liter of ethylene dichloride, and 0.54 pound of anhydrous ferric chloride. This chloromethylation reaction is carried out overnight. The chloromethylated boards are then rinsed in methyl alcohol (or ethylene dichloride) and placed overnight into a solution containing about 25% by volume or trimethylamine in water. The resulting aminated boards are removed from the trimethylamine, placed in 1 N HCl bath, washed with tap water to remove amine hydrochloride salts and then exchanged in 2 N NaCl solution followed by a thorough rinse with tap water to remove residual sodium chloride salts.

The resistivity of three small samples of the resulting anion membranes is determined at 1000 cycles in 0.01 N NaCl to give an average resistivity of 43.2 ohm-cm. This value is more than double the normal values of the prior art gel type membranes of similar formulation. The transport number of the membranes when measured in a concentration cell (0.1 N NaCl vs. 0.05 N NaCl) is found to be 0.744 which is substantially lower than the usual gel type membrane values of 0.95-0.98.

EXAMPLE II (Surface macroreticular membranes) 55% divinylbenzene ml 1 352 Styrene ml 116 Ethyl alcohol (B.P.=78.5 C.) ml 200 Diethylbenzene (DEB) ml 332 Benzoyl peroxide grams 4.5

1 Contained 18 ml. of DEB,

The above clear liquid mixture is placed into a polymerization tray with glass plates and sheets of woven Dynel cloth and polymerized as in Example I. The glass plates are then removed one at a time with concurrent release of the finished membranes. It is noted (and easily identified by odor water solubility test) that the poor solvent or ethyl alcohol has collected at the surfaces of the glass casting plates. Thus the poor solvent is generally located at the surfaces of each resulting macroporous surface board structure.

The resulting boards are immersed in diethylbenzene and are noted to be opaque in surface appearance in contrast to the clear gel type (nonmacroreticular) membranes. The boards having a macroporous opaque surface area and a clear gel type center are then chloromethylated and aminated to form anion exchange membranes as previously described.

In Example I where 78% of the non-polymerizable solvent used is the poor solvent of isobutyl alcohol, the membrane opaqueness is evidenced substantially throughout the board structure. However, in this Example 11 the macroreticular character of the board is located at the surfaces only thus its transport number of 0.85 is much higher than the value found for the totally macroreticular anion membrane structure of Example I.

Thus, unlike the totally macroreticular ion exchange membranes, this membrane has a gel type resin center, that is a center polymerized in the presence of a nonvolatile, good solvent and having pore sizes of between about 10-30 angstrom diameter and resin surfaces forced to polymerize in the presence of a volatile, poor solvent forming pore diameter in the range of about 500-1500 angstrom units.

These surface opaque anion membranes gave a resistivity of 26.6 ohm-cm. This compares to a resistivity of 1618 ohm-cm. for a corresponding totally gel type membrane of the same formulation using a preponderence of good solvent as the non-polymerizable diluent.

EXAMPLE III The same method of membrane fabrication is carried out as in Example II, except the more volatile methyl alcohol is used as the poor solvent in substitution for isobutyl alcohol. It had previously been determined, that if the temperature of polymerization exceeded the boiling point of the poor solvent, the resulting membranes would have large bubbles at or near their surfaces thus the polymerization temperature was kept between 55 and 60 C.

Formulation 55% divinylbenzene ml 405 Styrene ml Diethylbenzene (good solvent) ml 330 Methyl alcohol (poor solvent) (B.P=64.7 C.) ml Benzoyl peroxide grams 13.5

1 Contains 20 ml. of DEB.

The resulting boards are chloromethylated and aminated as previously described to produce anion membranes having a resistivity of 31 ohm-cm. the usual thickness of 0.055 cm., and an anion transport no. of 0.85. The high resistance of these novel surface macroreticular membranes as compared to the gel type membranes of the same formulation (usually 16-18 ohm-cm?) is a further indication of a macroreticular membrane structure whose surfaces contain essentially all large pore type resin with a central or inner portion somewhere between the macro and microporous structure. As in the case of the surface macroreticular membranes of Example II these membranes also possessed the opaque surface appearance and were not optically clear.

EXAMPLE IV The same method of membrane fabrication is carried out as in Example II except that a high boiling sec-butyl alcohol is used as the poor solvent of the non-polymerizable solvent mixture with polymerizartion carried out at 6590 C. The volume percent of poor solvent in the total volume of nonpolymerizable components of the bulk polymer mixture is about 20%.

Formulation 55% divinylbenzene ml 405 Styrene ml 100 Diethylbenzene ml 396 Sec-butyl alcohol (B.P. 995 C.) ml 104 Benzoyl peroxide grams 4.5

1 Contains 20 ml. of diethylbenzene. The resulting boards are chloromethylated and aminated as previously described.

Properties Resistivity 27.1 ohm-cm. Transport number 0.83. Appearance Hazy surface but with enough clarity to still see through membrane.

The high resistivity of these membranes as compared to the gel type membrane of the same formulation and the lower transport number and hazy appearance show them to be definitively, surface macroreticular membranes.

The surfaces of these membrane are sand papered to remove the hazy surface resin almost to the supporting cloth. Its resistivity decreased substantially and measured 20.7 ohm-cm. proving quite conclusively, by geometries, that most of the membranes macroreticular character is located at or near the surface planes.

9 EXAMPLE v This example of membrane preparation is identical to Example IV, except that the volume percentage of poor solvent in the total volume of nonpolymerizable components of the polymerization mixture was reduced further to about Formulation 55% divinylbenzene ml 405 Styrene ml 100 Sec-butyl alcohol ml 78 Diethylbenzene ml 422 Benzoyl peroxide gms 4.5

1 Contains ml. of diethylbenzene.

Properties Resistivity 21.0 ohm-cm. Transport number 0.94. Appearance Only a very slight hazy surface and clear enough to easily see directly through membrane.

This example proves by the resulting physical and measured characteristics that membranes can still be made having a slight macroreticular surface when the volume percentage of poor solvent is down to only about '15 of the total volume of poor and good solvents employed in the bulk polymerization mixture.

EXAMPLE VI The same method of membrane fabrication is carried out as in Example II except that 96.5% volume of poor solvent (octanoic acid) is used in the total volume of non-polymerizable components.

Formulation 55% divinylbenzene ml 1 405 Styrene ml 100 Octanoic acid (n-caprilic acid) (B.P.=239 C.) ml 500 Benzoyl peroxide grams 4.5

1 20 ml. of DEB.

The polymerization temperature is 65-80 C. with chloromethylation and amination as described in Example II.

Properties Resistivity 49.2 ohm-cm.

Anion transport 0.788.

Appearance Totally opaque throughout.

In this case a totally homogeneous macroreticular membrane result since essentially all of its non-polymerizable solvent is in the form of the poor solvent. A high resistivity and low transport number is noted for membranes prepared with substantially greater than 50% of its nonpolymerizable portion as poor solvent.

Example VII These membranes are prepared in the same general manner as Example I with the temperature of polymerization between 65-80 C.

The transport number is higher than the totally macroreticular membranes which indicates that only surface macroreticularity has been achieved, i.e. the center of the membrane still contains some gel form resin.

The following table summarizes the results of the previously described examples.

It has been further shown by laboratory experimentation including the examples herein described that surface macroreticular membranes may be prepared only where there is no less than a minimum of about 15% by volume of poor solvent present in the total volume of inert nonpolymerizable solvent up to a maximum not much greater than about 55% by volume of poor solvent. At below the 15 volume figure a substantially gel type membrane throughout is obtained offering no effective surface pore diameter increase and hence little resistance to membrane fouling. At much above the 55% volume level there is obtained a substantially total homogeneous macroreticular membrane whose surfaces and center are essentially indistinguishable as to porosity characteristics. Within the 15 to 55 range surface macroreticular membranes will be produced having resistivity measurements between about 20-40 ohm-cm. and anion transport numbers between about 0.80-0.94. These membranes will not only possess a non-fouling, highly porous surface but will additionally retain the desirable characteristic of a center or inner resin portion of the gel type microporous structure.

The embodiments of this invention in which an exclusive property or privilege is claimed are defined as follows:

1. An electrochemical cell comprising chambers adapted to contain liquid defined :by ion-permeable membranes and having terminal electrodes for passing a direct current transversely through said chambers and membranes, the improvement wherein some of said membranes are comprised of an ion-selective resin having at least a surface comprised of a macroporous structure having pore diameter sizes between about 500-1500 angstroms with the remaining portion of said membrane having a porosity substantially smaller than that of said macroporous structure.

2. An electrochemical cell comprising chambers adapted to contain liquid defined by ion-permeable membranes and terminal electrodes for passing a direct current transversely through said chambers and membranes, the improvement wherein at least some of said membranes being of the ion-selective type comprising a cross-linked, polyvinyl, polymeric resin sheet of a copolymer of at least one monovinyl aromatic monomer and at least one polyvinyl aromatic cross-linking monomer of which at least a surface of said polymeric sheet is of a macroporous structure having pore diameter sizes between about 500-1500 angstroms with the remaining portion of said membrane having a porosity substantially smaller than that of said macroporous structure.

3. In the method for reducing the degree of fouling of ion selective membranes during the electrolytic treatment of liquids containing membrane fouling constituents in an apparatus having a plurality of spaced ion-permeable membranes between terminal electrodes to define liquid containing chambers and wherein a direct current is passed transversely through membranes and liquid, the improvement wherein at least some of the membranes are comprised of an ion-selective resin having a macroporous structure located at least on the surface area side of said ion-selective membrane which is being subjected to the said fouling constituents, said macroporous structure having pore diameter sizes between about 500-1500 angstroms with the remaining portions of said membrane having a porosity substantially smaller than that of said macroporous structure.

References Cited UNITED STATES PATENTS JOHN H. MACK, Primary Examiner 10 A. C. PRESCOTT, Assistant Examiner U.S. Cl. X.R. 

